Beam-column joints of "ductile concrete" for seismic-resistant structures are required to flex and absorb large amounts of energy during an earthquake. Frequently, the flexure is severe enough to cause concrete to crack and spall out of joints. Spalling in the joint can result in failure of the joint, ultimately leading to building collapse. To ensure that seismic joints maintain integrity through several cycles of reversed flexure, current practice is to place a maze of intersecting bars and hoop ties in the joint. Because of the amount of steel used, such joints show a ductile response to loading and the concrete is called "ductile concrete"..sup.1 The congestion produced by the steel, particularly the hoop ties, increases labor costs on the order of $100 per joint over ordinary joints and complicates concrete placement.
An object of the present invention is to provide a ductile concrete joint that minimizes the steel congestion common to such joints. An experimental comparison was conducted on two full-sized building frame sections--one using a conventional design in accordance with the latest seismic-resistant design specifications of the American Concrete Institute (ACI-318-71) and the other using a modified design which incorporated steel fibrous concrete in the critical stress region and used less reinforcing steel. In the joint region where the fibrous concrete was used, all of the column hoop ties were eliminated.